Agitation tanks which contain particles and which receive a flow of a slurried pulp or slurry having metal or other values to be adsorbed on the particles have been used in the past with screen structures to keep the particles in the tanks as the slurry flows through and out of the tanks. In one particular application, namely in a carbon-in-pulp process, a slurry containing metal or other values is moved through a series of agitation tanks so that the values can be separated from the slurry by adsorption of the values by particles, such as carbon or resin particles, initially placed in the tanks. It is necessary that the particles be kept in the tanks as the slurry flows out of the tanks so that there will be continued adsorption of the values as the slurry continues to flow into and through the tanks. The carbon may be advanced from tank to tank counter-current to the flow of pulp, either on a batch or a continuous basis, with fresh carbon introduced to the last tank, and loaded carbon removed from the first tank, the loaded carbon being directed to another process step for recovering the values from the carbon.
In conventional carbon-in-pulp processing, each of a series of agitated tanks has a screen unit which allows passage of the slurry from one tank to another, yet the screen unit restricts the carbon particles with the metal and other values adsorbed thereon in one tank from passing out of the tank into another tank. Many different screen configurations have been developed and used with screen units of this type, including exterior mechanically and electrically vibrated screen units. The slurry must be delivered to these screens by pumps or similar devices. Other types of screen units include screens forming parts of the sides of overflow gravity launders.
All of these conventional screen units have disadvantages in that they either require mechanical or electrical vibrations or slurry transfer devices or they are difficult to clean and are not easily replaced. For instance, in U.S. Pat. No. 4,251,352 which describes a carbon-in-pulp process, a number of inclined screen segments are secured together at the upper end of a tank containing a slurry and carbon particles to adsorb metal and other values carried by the slurry. These screen segments frequently wear out and must be replaced. Because of their size and their locations with respect to each other in the tank, the replacement of the screens becomes a tedious job and one which requires considerable time and expense.
Because of the drawbacks of conventional agitation tanks described above, a need has arisen for improvements in such tanks to minimize the cost of making the tanks and the maintenance thereof.